This invention relates to dried grapes and specifically to a process of drying grapes in a vacuum using microwave energy or thermal radiation to produce a hollow dried grape of generally spherical configuration with an intact skin. Preferably the grapes are dried completely in a non-oxidizing atmosphere to preserve their original skin color and to produce a unique taste.
Raisins are dried grapes in which the meat and skin of the grape have collapsed around the center of the grape as it is dried to produce the wrinkled, prune-like appearance of the conventional raisin. Conventional raisins are made from Thompson seedless grapes. The grapes are green to yellow in color when picked for drying into raisins. Usually the grapes are dried in the field in rows using the heat of the sun. As the grapes dry, they oxidize and turn to the recognizable black or dark brown color normally associated with raisins. At the same time as the grapes dry, they collapse upon themselves around the center of the grape to produce the small, irregularly shaped wrinked raisin.
There are light colored raisins, known as golden raisins, and these are made by bleaching. The grapes are exposed to concentrated sulfur dioxide vapors. They are then dried artifically. The residual sulfur dioxide inhibits oxidation during the drying process and produces the "golden" product.
Drying grapes in the field makes them subject to the vagaries of the weather and some growers place the picked grapes under a shelter and use hot air to dry the grapes. Neither of these methods is totally satisfactory and both result in a black, inwardly, collapsed, wrinkled raisin.
The present invention produces a puffed or substantially spherical dried grape which is hollow but has approximately the original grape shape. All of the solids in the grape are present and the skin is substantially intact. The color is much lighter than that of a raisin. The dried grape has about 90% or more of the moisture removed and has a sweet, tart flavor. It can be made either chewy or crisp and brittle depending on the temperature to which it is heated and the time for which it is heated. The grape solids are mostly positioned against the inside of the skin.
The dried grape product preferably is produced by drying the grape in a vacuum using microwave energy or thermal heat radiation.
Placing the moist grapes in a vacuum chamber and reducing the chamber pressure causes the moisture to be evolved from the grapes and causes the primary constituent of the atmosphere to be water vapor, with no free oxygen for oxidizing the grapes. Either microwaves or thermal radiation are used to provide the energy input to cause the water to leave the grapes.
The art in Webb U.S. Pat. No. 2,110,184 describes a process of drying fruit or other food products in a duo drying process with a puffing or swelling of the fruit during the second drying step in an effort to retain the original flavor of the fruit. In the Webb process, the fruit initially is treated at a vacuum of 29.5 inches of mercury and a temperature of 300.degree.-320.degree. F. for 20-30 minutes until the temperature of the fruit is 180.degree.-200.degree. F. Then steam or other vapor pressure is admitted to the drying chamber to raise the pressure to 30-45 p.s.i. This is followed by a rapid evacuation of the chamber and the fruit is maintained at 180.degree.-190.degree. F. until it reaches a dry crystalline state. This is stated to take 30-60 minutes. The chamber then is rapidly cooled to cool the fruit to 100.degree.-125.degree. F. to crystallize the sugar and cause the fruit to harden. When the fruit is 100.degree.-125.degree. F. the chamber is gradually pressurized to atmospheric.
Moore U.S. Pat. No. 2,023,536 discloses a process for drying fruit to about 2% moisture. The Moore process involves drawing a vacuum of 25 inches of mercury followed by a steam treatment at 10-15 p.s.i.g. (115.degree. C.) for 2-3 minutes. The pressure is released and a vacuum drawn resulting in a "puffing action" on the fruit or an enlargement of the pores in the fruit. The heat is maintained to reduce the moisture to a desired low level. When whole fruit is used, i.e., prunes, grapes, product is macerated and pressed into cakes for drying. This type of dried fruit is good for use in baking, etc., often being broken into small chips.
Both of the foregoing processes had the disadvantage of requiring expensive equipment and the processes are time consuming, difficult and expensive to operate. The present invention provides a process for producing a dried whole fruit of the same substantial shape as the starting material in an efficient economical process involving only the use of a vacuum chamber combined with microwave energy or thermal radiation as a heat source.
Accordingly, one of the principal objects of the present invention is to provide an economical practical process of producing a dried whole grape having substantially the shape of the original grape with an intact skin using a vacuum drying process.
Still another object of this invention is to provide a process for making a dried non-oxidized grape having a tart sweet flavor and having the original grape shape.
A further object is to provide a process for making a dried substantially spherical shaped whole grape which can be either chewy or crisp depending on the temperature and time of which it is dried.
A still further object is to provide a vacuum drying process using microwave energy or thermal radiation for drying grapes and producing a hollow spherical shaped non-oxidized final product.
These and other objects and advantages will become apparent hereinafter.